A vacuum gauge is an important tool in diagnosing engine performance and checking vacuum operated accessories in a vehicle. The most common such gauge currently being used employs a so-called bourdon tube as its sensitive element. The tube is thin walled, flattened and bent to a circular arc which tends to be straightened as the vacuum increases. Such a gauge has a circular dial and a rotating pointer. A hose is coupled between the intake manifold of the engine and the gauge. The pointer will reflect the amount of vacuum present in the intake manifold measured in, for example, inches of mercury or PSI. The pistons in the engine serve as suction pumps, and the degree of vacuum they create is affected by the related actions of piston rings, valves, ignition, carburetor, etc. Each has a characteristic effect on the vacuum and their performances can be judged by watching for variations from normal. Not only is it important to know the average value of the vacuum as indicated by the gauge, but also of significance is the manner in which it fluctuates about such average value. A bourdon gauge is capable of giving information on the average vacuum and its fluctuation about such average value.
Of course, there are a number of disadvantages associated with any mechanical device. Average value can only be estimated since the needle is constantly moving. Also, a mechanical gauge is more susceptible to damage and is of generally less reliablility.
An electronic gauge that digitally displays the average magnitude of the vacuum is not the answer because it does not give information on the manner in which the vacuum fluctuates about such average.
Others have proposed devices which electronically produce information on average value by use of a digital display, and fluctuations about that value by use of a bar display. U.S. Pat. No. 3,684,870 to Nelson and U.S. Pat. No. 3,726,250 to Merk disclose examples, although they are not directed to measurement of vacuum. Both suffer the disadvantage that fluctuations in the bar graph are difficult to perceive. For example, if the amplitude of the condition measured by the gauge in either patent fluctuated between say 150 units and 175 units, the user would find it difficult to perceive an amplitude other than 175, because latency in the display would make it difficult for him to perceive that the minimum of the fluctuation was 150.
The bar amplitude should follow fluctuations in vacuum amplitude as closely as possible. The slower the display response, the less likely the user will be able to perceive meaningful fluctuations. The bulbs used in the Merk indicator and the liquid crystal devices used in the Nelson bar display do not respond fast enough.
While the foregoing background has been described in the context of a means to measure the magnitude of the vacuum in the intake manifold of an engine, the same general principles are applicable to measuring pressure in any chamber where not only its magnitude is important but so too is the manner in which its magnitude changes.